Method for preparing magnetic powder

ABSTRACT

The present invention relates to a method for preparing magnetic powder comprising homogeneous and fine particles using an alkali-producing enzyme. The object of the present invention is to provide a method suitable for preparing magnetic powder comprising relatively small particles, for instance, fine particles having a particle size ranging from 50 to 500 nm. The present invention relates to a method for preparing at least one member selected from the group consisting of iron oxides, iron hydroxides and iron oxyhydroxides which comprises the step of alkalizing a solution containing iron ions utilizing an alkali-producing enzyme and a substrate of the enzyme. 
     According to the present invention, there can be produced magnetite (Fe 3  O 4 ) and maghemite (gamma-Fe 2  O 3 ) useful as magnetic powder as well as goethite (alpha-FeO(OH)), hematite (alpha-Fe 2  O 3 ) and lepidocrocite (gamma-FeO(OH)) useful as the starting materials thereof. The magnetic powder can be used as the materials for magnetic recording and magnetic fluid; carriers for bioreactors; those for magnetic separation of cells and biopolymers; and those for microcarriers of medicines.

FIELD OF THE INVENTION

The present invention relates to a method for preparing magnetic powderand more particularly to a method for preparing uniform fine particlesof magnetic powder utilizing an alkali-producing enzyme.

BACKGROUND OF THE INVENTION

A method which comprises adding a precipitant to a metal salt solutionto form precipitates of a metal hydroxide has been used to preparemagnetic powder. However, when the precipitant is externary added tosuch a solution, the concentration of the precipitant in the solutionbecomes instantaneously and locally high and the resultant precipitatesare likely to cause impurities uptake even if the concentration of theprecipitant is low and it is added in small portions with stirring. Onthe contrary, use of a homogeneous precipitation method makes itpossible to provide pure and compact precipitates. This homogeneousprecipitation method is a method in which a precipitant is graduallyproduced in the solution by a chemical reaction such as hydrolysis.According to this method, the concentration of the precipitant is alwaysmaintained at an extremely low level since the produced precipitant isimmediately consumed. As a result, the incorporation of impurities intothe resultant precipitates and the amount of bound water are maintainedat a low level because the precipitated particles slowly grow with timeand precipitates of small volume is prepared.

Among the homogeneous precipitation methods, the most frequently used isan urea method in which a metal hydroxide or oxide is prepared utilizinga hydrolysis reaction of urea. The urea method is used to prepare ahydroxide or an oxide of iron.

However, it is found that the particle size of powder of such an oxideor the like prepared by the homogeneous precipitation method is large.Accordingly, an object of the present invention is to provide a methodwhich is suitable to prepare magnetic powder consisting of a relativelysmall particles such as fine particles having an average particle sizeranging from about 50 to 500 nm.

DISCLOSURE OF THE INVENTION

The present invention relates to a method for preparing at least onemember selected from the group consisting of iron oxides, ironhydroxides and iron oxyhydroxides, which comprises a step of alkalizinga solution containing iron ions using an alkali-producing enzyme and asubstrate thereof.

BRIEF EXPLANATION OF THE ACCOMPANYING DRAWINGS

FIGS. 1 and 3 to 7 are X-ray diffraction patterns and FIG. 2 is aphotograph illustrating the particle structure of the product obtainedin Example 1.

MOST PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present invention will now be explained in detail.

In the method of the present invention, a solution containing iron ionsis used. The solution contains either ferric or ferrous ions or both ofthese. The sources of these ferric and ferrous ions are not restrictedto specific ones and specific examples thereof include chlorides,sulfates, nitrates, lactates, oxalates, fumalates, ammonium sulfates andammonium oxalates of iron (including both ferric and ferrous salts).

In the method of the present invention, the aforementioned solutioncontaining iron ions is alkalized with an alkali-producing enzyme and asubstrate thereof.

The term "alkali-producing enzyme(s)" used herein means an enzymecapable of forming an alkali substance through a reaction with thesubstrate thereof and, for instance, hydrolases and oxidoreductases canbe used. Further, the invention encompasses a method in which thesolution containing iron ions is alkalized using a bacterial cellcapable of producing an alkali-producing enzyme. Examples of hydrolasesand substrates thereof which can be used in the invention are listed inTable I and oxidoreductases and substrates thereof are shown in TableII. In addition, bacteria which produce urease serving as analkali-producing enzyme are exemplified in Table III and examples ofother bacteria capable of producing an alkali-producing enzyme are shownin Table IV.

                  TABLE I                                                         ______________________________________                                        Hydrolases and Substrates Thereof                                             Enzyme No.                                                                             Name of Enzyme  Substrate                                            ______________________________________                                        3.5.1.1  asparaginase    L-asparagine                                         3.5.1.2  glutaminase     L-glutamine                                          3.5.1.3  omega-amidase   omega-amidodicar-                                                             boxylate                                             3.5.1.4  amidase         monocarboxylate                                      3.5.1.5  urease          urea                                                 3.5.1.6  beta-ureidopropionase                                                                         N-carbamoyl-beta-                                                             alanine                                              3.5.1.7  ureidosuccinase N-carbamoyl-L-alanine                                3.5.1.12 biotinidase     biotinamide                                          3.5.1.19 nicotinamidase  nicotinamide                                         3.5.1.20 citrullinase    L-citrulline                                         3.5.1.29 alpha-(acetamidomethy-                                                                        alpha-(acetamidome-                                           lene)-succinate thylene)-succinate                                            hydrolase                                                            3.5.1.30 5-aminovaleramidase                                                                           5-amino-n-valeramide                                 3.5.1.35 D-glutaminase   D-glutamine                                          3.5.1.38 glutaminase (aspara-                                                                          L-glutamine/L-                                                ginase)         asparagine                                           3.5.1.43 peptidyl-glutaminase                                                                          alpha-N-peptidyl-L-                                                           glutamine                                            3.5.1.44 glutaminyl-peptide                                                                            L-glutaminyl-peptide                                          glutaminase                                                          3.5.1.45 urease (ATP-hydroly-                                                                          urea                                                          zing)                                                                3.5.3.5  formiminoaspartate                                                                            N-formimino-L-aspar-                                          deiminase       tate                                                 3.5.3.6  arginine deiminase                                                                            L-arginine                                           3.5.3.9  allantoate deiminase                                                                          allantoate                                           3.5.3.12 agmatine deiminase                                                                            agmatine                                             3.5.3.13 formiminoglutamate                                                                            N-formimino-L-                                                deiminase       glutamate                                            3.5.4.1  cytosine deaminase                                                                            cytosine                                             3.5.4.2  adenine deaminase                                                                             adenine                                              3.5.4.3  guanine deaminase                                                                             guanine                                              3.5.4.4  adenosine deaminase                                                                           adenosine                                            3.5.4.5  cytidine deaminase                                                                            cytidine                                             3.5.4.6  AMP deaminase   AMP                                                  3.5.4.7  ADP deaminase   ADP                                                  3.5.4.8  aminoimidazolase                                                                              4-aminoimidazole                                     3.5.4.11 pterin deaminase                                                                              pterin                                               3.5.4.12 dCMP deaminase  dCMP                                                 3.5.4.13 dCTP deaminase  dCTP                                                 3.5.4.14 deoxycytidine deaminase                                                                       deoxycytidine                                        3.5.4.15 guanosine deaminase                                                                           guanosine                                            3.5.4.17 adenosine phosphoric                                                                          AMP                                                           acid deaminase                                                       3.5.4.18 ATP deaminase   ATP                                                  3.5.4.20 pyrithiamin deaminase                                                                         pyrithiamine                                         3.5.4.21 creatinine deiminase                                                                          creatinine                                           3.5.4.22 1-pyrroline-4-hydroxy-2-                                                                      HPC                                                           carboxylate deaminase                                                3.5.4.23 blasticidin-S deiminase                                                                       blasticidine-S                                       3.5.4.24 cepiapterin deaminase                                                                         cepiapterin                                          3.5.5.1  nitrilase       3-indole-acetonitrile                                3.5.5.2  ricinine nitrilase                                                                            ricinine                                             3.5.5.3  cyanate hydrolase                                                                             cyanate                                              ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Oxidoreductase and Substrates Thereof                                         Enzyme No.                                                                              Name of Enzyme   Substrate                                          ______________________________________                                        1.6.6.4   nitrite reductase                                                                              nitrite                                                      (NAD(P)H)                                                           1.6.6.9   trimethylamine-N-oxido-                                                                        trimethylamine-N-                                            reductase        oxide                                              1.6.6.11  hydroxylamine reductase                                                                        hydroxylamine                                                (NADH)                                                              1.7.7.1   ferredoxin-nitrite                                                                             nitrite                                                      reductase                                                           1.7.99.1  hydroxylamine reductase                                                                        hydroxylamine                                      1.18.2.1  nitrogenase      N.sub.2                                            1.19.2.1  nitrogenase (flavodoxin)                                                                       N.sub.2                                            ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Urease (EC 3.5.1.5)-producing Bacteria                                        Proteus vulgaris (ATCC 13315)  Proteus columbiensis                           Proteus morganii (ATCC 25830)  Micrococcus                                    Aerobacter aerogenes                                                          Klebsiella pneumoniae (ATCC 13883)                                            Pseudomonas aeruginosa (ATCC 10145)                                           Escherichia coli (ATCC 11775)                                                 Alcaligenes faecalis (ATCC 8750)  Salmonella                                  Vibrio cholerae (ATCC 14035)  Vibrio El Tor                                   Shigella   Eberthella  Enterococcus                                           Pneumococcus  Neisseria                                                       Streptococcus pyogenes (ATCC 21060)                                           Lactobacillus acidophilus (ATCC 11506)                                        Lactobacillus bulgaricus (ATCC 11842)                                         Lactobacillus doderlenii                                                      Brucella melitensis (ATCC 23456)                                              Brucella abortis (ATCC 23448)  Pasteurella pestis                             Bacillus subtilis (ATCC 6633)                                                 Mycobacterium tuberculosis (ATCC 25618)                                       ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        Asparaginase (EC 3.5.1.1)-producing Bacteria                                  Escherichia coli (ATCC 11775)                                                 Erwinia carotovora (ATCC 25206)                                               Bacillus coagulans (ATCC 12245)                                               Fusarium tricinctum (ATCC 24631)                                              Proteus vulgaris (ATCC 13315)                                                 Saccharomyces cerevisiae (ATCC 26923)                                         Glutaminase (EC 3.5.1.2)-producing Bacteria                                   Pseudomonas aeruginosa (ATCC 27107)                                           Escherichia coli (ATCC 11775)  yeast  Actinomyces                             Adenine Diaminase (EC 3.5.4.2)-producing Bacteria                             Azotobacter vinelandii (ATCC 25308)                                           Candida utilis (ATCC 9950)                                                    Crithidia fasticulata (protozoan)                                             Guanine Deaminase (EC 3.5.4.3)-producing Bacteria                             Clostridium  Pseudomonas                                                      Adenosine Deaminase (EC 3.5.4.4)-producing Bacteria                           Pseudomonas                                                                   AMP Deaminase (EC 3.5.4.6)-producing Bacteria                                 yeast                                                                         ______________________________________                                    

According to the present invention, at least one member selected fromthe group consisting of iron oxides, iron hydroxides and ironoxyhydroxides can be prepared by alkalizing a solution containing ironions utilizing an alkali-producing enzyme and a substrate thereof.

Products obtained by alkalization vary depending on the reactionconditions. The reaction conditions herein mean the kind of iron ions(whether they are Fe²⁺ or Fe³⁺), ratio (molar ratio) of theconcentration of Fe²⁺ to that of Fe³⁺, the amount of enzymes andsubstrates thereof used, the state of the reaction system (whether it isoxidation or reduction system), temperature and the like.

More specifically, the method of the present invention makes it possibleto prepare powder such as Fe₃ O₄ (magnetite), alpha-FeO(OH) (goethite),beta-FeO(OH) (akaganeite), gamma-FeO(OH) (lepidocrocite), delta-FeO(OH),alpha-Fe₂ O₃ (hematite), gamma-Fe₂ O₃ (maghemite), Fe(OH)₂ and Fe(OH)₃.

(a) Cases Where the Solution Containing Iron Ions Contains Fe²⁺ as IronIons:

In such a case, the alkalization of the foregoing solution results inthe formation of Fe(OH)₂. In this connection, Fe(OH)₂ is relativelyunstable. Therefore, in the present invention, the akalization iscarried out in the presence of an oxidizing agent or an oxidizing agentis added to the reaction system after the alkalization to completely orpartially oxidize Fe²⁺ to Fe³⁺ and magnetite or an iron oxyhydroxide areobtained. Products obtained by such an oxidation vary depending on theconditions thereof such as the oxidation temperature, molar ratio ofiron ions to the substrate present in the solution, the degree ofprogress of the reaction and Fe₃ O₄, a mixture of Fe₃ O₄ andalpha-FeO(OH), alpha-FeO(OH) or a mixture of alpha-FeO(OH) andgamma-FeO(OH) can be prepared. The relation between the oxidationconditions of Fe(OH)₂ and the kinds of the products is disclosed in FIG.1 attached to 1st to 25th reports on Water-containing Iron Oxides,Toshio TAKADA & Masao KIYAMA, Collected Summary for Lectures on Powderand Powder Metallurgy and in the method of the present invention, thekind of the product can be selected on the basis of the data plotted onthe figure.

For instance, if the reaction temperature ranges from about 10° to 30°C., the oxidation products of Fe(OH)₂ are magnetite and alpha-FeO(OH)for a molar ratio of Fe²⁺ to OH⁻ of 1: about 2; alpha-FeO(OH) for themolar ratio of 1: about 3 or less; and a mixture of alpha-FeO(OH) andgamma-FeO(OH) for the molar ratio of 1: about 1.5 or less. Moreover,when the reaction temperature is not less than 50° C., magnetite isobtained at a molar ratio of Fe²⁺ to OH⁻ of 1: about 1 or more.

The oxidizing agent for oxidizing Fe²⁺ is not restricted to specificones and examples thereof include oxygen, ozone, manganese dioxide, leaddioxide, permanganates, chlorates, hydrogen peroxide, sodium peroxide,ferric salts and nitrates (sodium and potassium salts).

The resulting alpha-FeO(OH) and gamma-FeO(OH) can be converted tohematite (alpha-Fe₂ O₃) and maghemite (gamma-Fe₂ O₃) respectively bydehydrating them according in an ordinary method.

(b) Cases Where the Solution Containing Iron Ions Contains Fe²⁺ and Fe³⁺as Iron Ions:

When the solution containing iron ions contains Fe²⁺ and Fe³⁺ as ironions, magnetite can be obtained by adjusting the molar ratio of Fe²⁺ toFe³⁺ to the range within which magnetite is formed, before thecompletion of alkalization or the end of the formation of magnetite.Magnetite can be formed at a molar ratio of Fe²⁺ to Fe³⁺ ranging, forinstance, from 1: about 0.5 to 5, preferably 1: about 1 to 3.

When Fe²⁺ is present in excess compared with the foregoing range, themolar ratio of Fe²⁺ to Fe³⁺ can be adjusted by carrying out thealkalization in the presence of an oxidizing agent or adding anoxidizing agent to the reaction system before the complete formation ofmagnetite. On the contrary, when Fe³⁺ is present in excess compared withthe foregoing range, the molar ratio of Fe²⁺ to Fe³⁺ can be controlledby carrying out the alkalization in the presence of a reducing agent oradding a reducing agent to the reaction system before the completeformation of magnetite.

The oxidizing agents used to adjust the molar ratio, Fe²⁺ /Fe³⁺, are notrestricted to specific ones and examples thereof are those mentionedabove in connection with item (a). In addition, the reducing agents arenot restricted to specific compounds either and include, for instance,hydrogen, ferrous salts and titanium (Ti²⁺, Ti³⁺) in low oxidizedstates.

Upon alkalizing the solution containing Fe²⁺ and Fe³⁺ in the proceduresdescribed above, iron hydroxide is first formed and the molar ratio,Fe²⁺ /Fe³⁺, is adjusted if necessary, the iron hydroxide formsprecipitates (hydrophobic colloid). From the hydrophobic colloid,coagulation occurs on standing and as a result the crystals of magnetiteis formed. This processes are as follows: ##STR1##

The term "magnetite" herein means not only those represented by thechemical formula: Fe₃ O₄ but also iron oxides which contain ferric andferrous irons and exhibit magnetic properties.

Moreover, when the solution contains Fe²⁺ and Fe³⁺ as iron ions and themolar ratio, Fe²⁺ /Fe³⁺, is outside the range within which the magnetiteis formed, for instance, Fe(OH)₂ is principally produced in the casewhere Fe²⁺ is present in excess and Fe(OH)₂ is converted to magnetite orthe like depending on the conditions as already mentioned above inconnection with item (a). When Fe³⁺ is present in excess, Fe(OH)₃ ismainly produced as will be explained in item (c) given below and Fe(OH)₂derived from Fe²⁺ is sometimes converted to magnetite as in item (a)depending on the concentration of Fe²⁺.

(c) Cases Where the Solution Containing Iron Ions Contains Fe³⁺ as IronIons

In such a case, Fe(OH)₃ is produced as the product of the alkalizationreaction. Fe(OH)₃ can be reduced according in an ordinary manner toobtain magnetite. The reducing agents used herein are not restricted tospecific ones, but those described above in item (b) can be used. As tothe degree of reduction, it is preferable to limit the molar ratio, Fe²⁺/Fe³⁺, to, for instance, 1: about 0.5 to 5, preferably 1: about 1 to 3,within which magnetite can be obtained.

Magnetite may also be obtained by carrying out the alkalization in thepresence of a reducing agent.

The conditions of the alkalization in the method of the presentinvention will hereunder be explained.

In the present invention, the alkalization is initiated by adding, to asolution containing iron ions, an alkali-producing enzyme or bacteriacapable of producing an alkali-producing enzyme and a substrate of theenzyme. The alkalization is preferably performed in the vicinity of theoptimum conditions of the alkali-producing enzyme and the optimumconditions vary depending on the kind of the enzyme used. For instance,when urease derived from jack bean is used as such an alkali-producingenzyme, the alkalization is desirably carried out at a temperatureranging from about 10° to 60° C., preferably about 25° to 50° C.; and aninitial pH of 4.5 to 6.7, preferably 6.0 to 6.7. When asparaginase isused as such an alkali-producing enzyme, the temperature ranges from 10°to 45° C., preferably 35° to 40° C.; and the initial pH of thealkalization ranges from 6.0 to 6.7. In this connection, it should benoted that the initial pH value of the alkalization be in the rangewithin which the iron ions are not converted to hydroxides but are inthe dissolved state, in other word in the neutral or acidic range.

The time required for the alkalization depends on the concentrations ofiron ions, enzymes and substrates used; as well as the temperature ofthe reaction and the scale thereof, but it normally ranges from one hourto 5 days. In addition, the concentration of iron ions and the amount ofthe substrate and enzyme can be properly selected in the presentinvention. The concentration of iron ions in the solution is, forinstance, in the range of from 0.5 to 50 mmole/1, preferably 3 to 10mmole/1. The amount of the enzyme used is not less than 0.1 unit/ml,preferably not less than 10 unit/ml, while the amount of the substrateused is suitably 0.01 to 100 times, preferably 0.1 to 10 times the molarnumber of iron ions.

The alkalization reaction can be carried out in either a batchwise orcontinuous manner. Moreover, it is also possible to use analkali-producing enzyme or bacteria capable of producingalkali-producing enzyme which are immobilized according in an ordinarymanner.

Magnetite or the like obtained according to the method of the presentinvention is isolated from the solution and can be used as such afterwater washing or can be used as magnetic powder or a starting materialtherefor after subjecting it to a stabilization treatment(s). Examplesof such stabilization techniques include (i) a method which comprisesforming a thin oxide film on the particle surface (see Japanese PatentUn-examined Publication (hereinafter referred to as "J.P. KOKAI") No.49-97738); (ii) a method which comprises forming a film of a higherfatty acid on the particle surface (see J.P. KOKAI No. 49-97738); (iii)a method which comprises adhering an amino-modified silicone oil or thelike to the particle surface (see J.P. KOKAI No. 54-77270); and (iv) amethod which comprises adhering a boron trialkoxide to the particlesurface.

According to the present invention, there can be produced magnetite (Fe₃O₄) and maghemite (gamma-Fe₂ O₃) useful as magnetic powder as well asgoethite (alpha-FeO(OH)), hematite (alpha-Fe₂ O₃) and lepidocrocite(gamma-FeO(OH)) useful as the starting materials for such magneticpowder. These magnetic powder can be useful as the materials formagnetic recording or magnetic fluid; carriers for bioreactors; thosefor magnetic separation of cells or biopolymers; those for microcarriersfor medicines or the like.

The method of the present invention belongs to the homogeneousprecipitation method and, therefore, this method makes it possible toobtain pure and compact precipitates and thus makes it possible toobtain fine magnetic powder. By varying the alkali-producing conditionsof the enzyme used, the particle size of the resulting powder can bechanged within the range of 50 to 500 nm and the particles can be formedinto various shapes such as needle, spherical or polygonal prismaticforms.

In addition, the reaction equipments are heavily consumed in recent wetmethods because of the use of an alkali such as sodium hydroxide orammonia. However, the alkalization reaction is carried out in thevicinity of neutral pH utilizing an enzyme or bacteria and, therefore,such a problem does not occur in the method of the present invention.

The present invention will hereunder be explained in more detail withreference to the following Examples.

EXAMPLE 1

100 ml of an aqueous solution including 8.9 mM of urea, 4.2 mM offerrous chloride and 0.2 M of potassium nitrate was charged into acontainer and was degassed with nitrogen gas. The container was sealedwith a butyl rubber plug and then was maintained at 25° C. To thesolution there was added 1 ml (1,000 units/ml) of an urease solutionwith a syringe and the solution was again maintained at 25° C. After 48hours, black precipitates were deposited on the bottom of the container.All the precipitates exhibited magnetic susceptibility. The precipitateswere examined by X-ray diffraction analysis and the results obtainedwere shown in FIG. 1. In addition, the electron micrograph thereof isshown in FIG. 2. These results indicate that the black precipitates arehomogeneous fine particles of magnetite having a diameter of 200 nm.

EXAMPLE 2

Using 100 ml of an aqueous solution containing 3.0 mM of urea, 1.4 mM offerrous chloride and 0.2 M of potassium nitrate, the reaction wascarried out as in Example 1 and as a result magnetite was obtained as inExample 1. The X-ray diffraction pattern thereof is shown in FIG. 3.

EXAMPLE 3

The same starting materials as those used in Example 1 were employed andair was passed through the solution in place of degassing the same withnitrogen gas. As a result, a mixture of lepidocrocite and akaganeite wasproduced. The X-ray diffraction pattern is shown in FIG. 4.

EXAMPLE 4

Using an aqueous solution containing 2.0 mM of urea, 4.2 mM of ferrouschloride and 0.2 M of potassium nitrate, the reaction was carried out asin Example 1 and as a result, lepidocrocite was obtained. The X-raydiffraction pattern thereof is shown in FIG. 5.

EXAMPLE 5

To 200 mg of an aqueous solution containing 200 mM of urea, 100 mg offerrous chloride hydrate (FeCl₂ -nH₂ O) and 0.1 M of potassium nitratethere was added 1 ml of urease solution (1,000 units/ml), then liquidparaffin was added thereto to form a liquid paraffin layer with thethickness of about 1 cm on the surface of the solution and the solutionwas kept to stand at 25° C. After 48 hours, precipitates of blackishbrown were deposited on the bottom of the container. The precipitateswere examined by X-ray diffraction analysis (see FIG. 6) and were foundto be maghemite.

EXAMPLE 6

Starting materials were dissolved into 10 ml of sufficiently degassedwater to form a starting solution having concentration of 80 mM of urea,5.0 mM of ferrous chloride and 2.0 mM of ferric chloride. The gas phaseand oxygen dissolved in the solution were removed and 1 ml of an ureasesolution (1,000 units/ml) was added thereto during it being allowed tostand in air. After 24 hours, magnetically susceptible blackprecipitates were formed. It was found that the precipitates weremagnetite as estimated from the color and the magnetic susceptibility.

EXAMPLE 7

The reaction was carried out as in Example 6 except that urea was usedin a concentration of 8.0 mM and ferric chloride in a concentration of2.4 mM. After 5 minutes, yellow gel-like colloid was obtained. To thecolloid ferrous chloride was added to make a concentration thereof 2.4mM. After 24 hours, magnetically susceptible black precipitates wereformed. The precipitates were magnetite as in Example 6.

EXAMPLE 8

Asparaginase (0.1 ml; 1,000 units/ml) was added to a solution containing10 mM of asparagine and 5 mM of ferrous chloride. The solution wasmaintained at 37° C. and two hours thereafter, the solution wascentrifuged in a centrifugal separator. As a result, yellow precipitateswere obtained on the bottom of a centrifugal tube, but these were notmagnetically susceptible. It was found that the precipitates werelepidocrocite as estimated from the color and the pH value (6.7) afterthe reaction.

EXAMPLE 9

The gas phase present in 100 ml of an aqueous solution containing 8.9 mMof urea, 4.2 mM of ferrous chloride and 0.2 M of potassium nitrate wasreplaced with nitrogen gas. The container of the solution was sealedwith a butyl rubber plug and then the solution was maintained at 25° C.On the other hand, P.vulgaris was cultured in 10 ml of a culture mediumin an ordinary manner, followed by collecting bacterial cells, washingthem with physiological saline and suspending the cells into 1 ml ofdistilled water. The suspension of bacterial cells was added to thesolution of a substrate prepared above with a syringe and the solutionwas again maintained at 25° C. After 48 hours, magnetically susceptibleblack precipitates were deposited on the bottom of the container. Theresulting precipitates were separated from the bacterial cells by theaction of a magnet and then washed with distilled water. Theprecipitates were examined by X-ray diffraction analysis (see FIG. 7)and were found to be magnetite.

We claim:
 1. A method for preparing at least one member selected fromthe group consisting of iron oxides, iron hydroxides and ironoxyhydroxides comprising: reacting an alkali-producing enzyme with asubstrate of the enzyme in a solution containing iron ions to form analkali substance, thereby alkalanizing the solution by the alkalisubstance; and obtaining fine particles of the iron compound having anaverage particle size ranging from about 50 to 500 nm.
 2. A method asset forth in claim 1 wherein the alkali-producing enzyme is a hydrolase.3. A method as set forth in claim 1 wherein the alkali-producing enzymeis an oxidoreductase.
 4. A method as set forth in claim 1 wherein thealkali-producing enzyme is urease (EC 3.5.1.5).
 5. A method as set forthin claim 1 wherein the alkali-producing enzyme is one prepared, in situ,by bacteria capable of producing an alkali-producing enzyme.
 6. A methodas set forth in claim 5 wherein the bacteria capable of producing analkali-producing enzyme is urease-producing bacteria.
 7. A method as setforth in claim 1 wherein the solution containing iron ions contains Fe²⁺as iron ions and the alkalization is carried out in the presence of anoxidizing agent to obtain magnetite and/or iron oxyhydroxides.
 8. Amethod as set forth in claim 7 wherein the iron oxyhydroxides aredehydrated to obtain hematite and/or maghemite.
 9. A method as set forthin claim 1 wherein the solution containing iron ions contains Fe²⁺ asiron ions and an oxidizing agent is added to the reaction system afterthe alkalization to convert whole or part of Fe²⁺ to Fe³⁺ by oxidationto thus obtain magnetite and/or iron oxyhydroxides.
 10. A method as setforth in claim 9 wherein the iron oxyhydroxides are dehydrated to obtainhematite and/or maghemite.
 11. A method as set forth in claim 1 whereinthe solution containing iron ions contains Fe²⁺ and Fe³⁺ as iron ionsand molar ratio of Fe²⁺ to Fe³⁺ is adjusted to a range within whichmagnetite is formed before the completion of the alkalization or beforethe end of the magnetite formation to obtain magnetite.
 12. A method asset forth in claim 11 wherein the molar ratio of Fe²⁺ to Fe³⁺ (Fe²⁺:Fe³⁺) at which magnetite is formed is in the range of 1:0.5 to
 5. 13. Amethod as set forth in claim 11 wherein the molar ratio of Fe²⁺ to Fe³⁺(Fe²⁺ :Fe³⁺) at which magnetite is formed is in the range of 1 to
 3. 14.A method as set forth in claim 1 wherein the solution containing ironions contains Fe³⁺ as iron ions to obtain iron hydroxides.
 15. A methodas set forth in claim 14 wherein the resultant iron hydroxides arereduced to form magnetite.
 16. A method as set forth in claim 1 whereinthe solution containing iron ions contains Fe³⁺ as iron ions and thealkalization is carried out in the presence of a reducing agent toobtain magnetite.